Wireless direction finding system



Nov.. 9, 1937.

WIRELESS DIRECTION FINDING SYSTEM H. M. DOWSETT ET AL Filed March 20. 1934 ATTORNEY Patented Nov. 9, 1937 UNITED STATES NT OFFICE WIRELESS DIRECTION FINDINGl SYSTEM of Delaware Application March 20,

1934, Serial vNo. 716,474

ln Great Britain March 31, 1933 3 Claims.

This invention relatesV to wireless direction nding and navigation systems and has for its object to provide an improved system wherein direction finding operations can be satisfactorily accomplished by relatively unskilled persons.

In wireless direction finding and navigationsystems at present in common use ai'lxed (shore) transmitting station transmits Morse or other code signals which are picked up by mobile receivers (ships) and serve lto identify said station. Information as to the direction or bearing of said station is obtained at the receiver in any of a variety of well known ways-e. g. where the transmitter is omnidirectional, by means of a 155 rotatable frame aerial .or radiogoniometer Search coil at said receiver. By reason of the use of the Morse or equivalent code, skilled operators are required for such systems. and the present invention provides an improved system wherein the need for such operators is avoided by replacing the code signals by what are commonly called video signals, because the modulation impulses thereof are representative of light and dark elemental areas of a scanned picture. The

l transmitters in our system are caused to send television pictures of their identification names and pictures of directional indications. The receivers, on the other hand, are fitted with television picture reconstructing devices.

The main object of this invention is to provide an improved television bearing finding or navigation system wherein synchronism between the transmitting and receiving television scanning devices shall be maintained and shall not be liable to be lost as a result of variations in received signal strength consequent upon the operation of bearing iinding.

As will be seen later herein, the invention provides a system wherein proper synchronism is maintained between transmitting and receiving scanning devices and wherein no skilled operators are required at the receiver at all, the receiver being of the omni-directional type.

According to this invention a wireless transmitting station for use for direction finding and navigation purposes transmits continuously a characteristic picture which may consist of or include the identification name of the station. The device comprises two frame aerials which are arranged at right angles to one another and are continuously rotated. One of these frame aerials is energized with a carrier frequency modulated with the television signal plus a synchro-- nizing signal and the other is energized by the carrier frequency modulated with the synchronizing signal only. The two carriers may be of the same frequency and derived from a common driving source. The polar diagram resulting from a transmitting arrangement in accordance with this invention and so far as the synchronizing signal modulated energy is concernedA will be a circle, or a close approximation thereto, and hence the receiving station (which has an omnidirectional antenna) will pick up a substantially constant synchronizing signal strength independent of the bearing of the transmitting station relative thereto. On the other hand, however, the polar diagram of transmission so far as the television signal modulated energy is concerned, will be-a rotating gure-of-eight diagram.

The nature of my invention and one manner in which the same may be carried out has been illustrated in the drawing, in which:

Fig. 1 shows diagrammatically the essential features of a transmitting system arranged in accordance with my invention;

Fig. 2 illustrates details of aerials of the dlrective type which may be used in the system in Fig. l; while Fig. 3 illustrates by curves the nature of the various fields produced and the manner in which the same are modulated.

A convenient particular arrangement of transmitting apparatus in accordance with this invention is illustrated in Figures 1 and 2 of the vaccompanying drawing and comprises a source I of synchronizing signals whose output is suitably amplied by a low frequency amplifier 2 and then applied to a modulator 3 which modulates as an intermediate amplifier 5 a local oscillation frequency derived from a source 4 of local oscillations, the resultant modulated carrier wave being further amplied as necessary at 6 and radiated from one (frame B) of two mutually perpendicular frame aerials A and B (shown in Figure 2) which are mounted to rotate together. The aerials are preferably mounted on a rotatable shaft as indicated conventionally in Fig. 2 by the arrow surrounding said shaft. Rotation may also be had by means of a continuously driven motor (not shown). The transmitting apparatus also comprises a source l of characteristic television signals which are amplied and combined at 9 with synchronizing signals obtained, after further amplification at 8, if required, from the synchronizing signal generator or (as shown) from the amplifier 2 energized therefrom. Means (not shown) are preferably provided for adjusting the phase and amplitude relations between the television and synchronizing signals before mixing them at 9. The mixed signals are fed to a modulator I which modulates in an intermediate amplifier I6 an oscillaulated carrier, after such further amplification at I3, as may be necessary, is impressed upon the second A of the mutually perpendicular for energizing one of said antenna systems with frames.

Means should, of course, be'provided to enable the officers at the receiving station to learn the direction of the transmitting station as a result of the reception of energy in the rotating figureof-eight transmission. Preferably this is accom- Y plished afterY the manner set forth in British Patent No. 400,279 by modulating the energy in the rotating gure-of-eight transmission with television signals giving, in addition to an indication identifying the transmitting station,` an indication of the direction of the rotating beam at any instant. y Thus, for example, the picture transmitted on the rotating beam may consist in part of a picture of the name of the transmitting station and in part of a portion of a compass card, said portion, of course, continuously changing as the beam rotates.

The general nature of the polar diagrams resulting from a transmitter as shown in Figures jl and 2 is illustrated in the accompanying Figure `Ysignal polar diagram will, of course, be the diagram a, since only frame A radiates the television signals. a is the diagram of frame A and Vl) the diagram of frame B.

Itwill be seen that the present invention pro-Y y .vides an arrangement wherein a compact ,frame aerial system may be employed, and wherein the desired Vphase and amplitude relations of the radiated waves carrying the television and synchronizing signals may be easily obtained so that Vthe synchronizing signal transmission does not 1. In a Wireless transmitting station for use for direction nding, television apparatus for transmitting television signals representative of a characteristic subject, including a directional indication, means for transmitting accompanying signals for synchronization, a plurality of directive antenna systems the directivity of which may be varied according to variations in the directional indications to be transmitted, means carrier energy modulated with the television signals, a source of synchronizing signals, and means for. energizing all of the said directive systems With carrier energy modulated with. said synchronizing signals.

2. In a directional indicating radio beacon system, means including a plurality of Variably directive antennae for radiating modulated signals, means forso varying the significance of the modulat-ed signals in accordancewith the directive effect of oneY of said antennae l as to produce distinctive indications appropriate to each direction, meansV including a panoramic picture to be scanned synchronously with the rotation of the directive effect of that one of said antennae previously mentioned whereby the modulated signals may be composed, and means for generating and for simultaneouslyV radiating from both said antennae non-directional'synchronizing signals whereby a television receiver may be maintained operative to reconstruct said distinctive directional indications.

3. In a radio beacon system, a'pair of loop antennae having a common vertical axis of rotation and maintained in mutually perpendicular planes,

' means for continuously rotating said antennae about lsaid axis, a source of carrier frequency energy, means including a source of synchronizing signals for modulating said carrier frequency energy, means for amplifying the energy so modulated and for causing both said antennae to radiate said energy simultaneously, a source of television signals, means for causing said television signals to be applied as additional modulations of the energyradiated from one only of said antennae, and means for characterizing said television signals in accordance with variations in the directivity of the antenna to which they are applied.

HARRY MELVILLE DOWSETT.

LOUIS EDWARD VQUINTRELL WALKER. 

